The present defects of this type of injection system for diesel engines lie firstly in a dispersion of engine characteristics on leaving the factory, e.g. due to manufacturing tolerances and to the extremely high pressures of the system, and secondly to drift over time in the delivery characteristics of the injectors, while the vehicle is in use. Furthermore, over its lifetime, the engine's own characteristics vary, for example the compression ratio in the cylinders, and the permeability of the valves or of the piston rings, thus altering the thermodynamic conditions of combustion. The sum of these drifts leads to degradation of the combustion noise of the engine that is perceived directly by the user of the vehicle.
In the state of the art, various systems are already known that seek to reset the injection system over the lifetime of the engine so as to ensure that the quantities injected correspond to the reference value. For example, patent EP 0 959 237 B1 presents a method of adjusting the time between top dead center and injection by using an accelerometer. Other methods are known for resetting pilot rates that are based on engine speed sensors.
Systems are also known to seek to modify the quantities of fuel injected into the cylinders by taking account of such drift in order to correct degradation of combustion noise on a regular basis. Those injection systems are adapted to inject fuel into the engine cylinders in the form of at least one pilot injection and a main injection.
The pilot injection serves to inject a small quantity of fuel a few milliseconds (ms) before the main injection fuel for the purpose of minimizing combustion noise. This small quantity of fuel favorably conditions the ignition characteristics of the quantity of fuel that is injected during main ignition by reducing the pressure gradients that are responsible for combustion noise.
Systems are known for controlling the operation of a diesel engine by controlling said pilot injection. Such systems propose modifying the quantities of fuel that are injected during pilot injection so as to minimize combustion noise on the basis of calculating a combustion noise score referred to as the combustion pressure energy index (CPEI) system. Nevertheless, such systems present several drawbacks since noise score calculation makes use of a model of the engine and a model of the human ear.
That assumes that the real behavior of the engine can be approximated by a mathematical model, which introduces errors. In order to reduce the amplitude of such errors, it becomes necessary to increase the complexity of the model, thereby having the consequence, not only of increasing computation time, but also of increasing the complexity of the system itself, in particular in terms of memory size and computation power of the on-board controller.
In addition, by the very nature of the problem, the engine model used can become obsolete since the characteristics of the engine change over time, implying that the model is no longer representative after a certain length of time, unless its parameters are regularly updated, which likewise involves an increasing degree of complexity.
Furthermore, using a model of the human ear implies the additional disadvantage of assuming that there is only one model that is applicable to the hearing behavior of all users.
That approach requires very long computation time since it makes use of complex digital filtering. In addition, the values obtained are dispersed very widely, which requires a large number of cycles to be averaged before a value can be obtained that is suitable for use in controlling injection.
In order to increase the reliability with which combustion noise is minimized, one solution then consists in making use of a different combustion noise score.
For this purpose, European patent application EP-A-1 209 458 relates to a method of determining combustion noise by using the wavelet method. That method does indeed avoid using a model of the human ear, and it does not make direct use of an engine model when calculating the noise score. However, that method remains difficult to apply in simple manner. That approach remains an approach that is based on a mathematical model of the engine, if only for the purpose of selecting the basis functions to use, and it is consequently subject to the same drawbacks as are associated with using a model. Finally, that approach remains expensive in computation time since it requires several successive digital filtering operations in order to obtain information that is useful for controlling injection. In addition, the dispersion in individual values is even greater than with the “CPEI” method.